The present invention relates generally to communication systems and more particularly relates to an apparatus for and a method of reducing interference in a communications receiver.
In recent years, the world has witnessed explosive growth in the demand for wireless communications and it is predicted that this demand will increase in the future. There are already over 500 million users that subscribe to cellular telephone services and the number is continually increasing. Eventually, in the not too distant future the number of cellular subscribers will exceed the number of fixed line telephone installations. Already, in many cases, the revenues from mobile services already exceeds that for fixed line services even though the amount of traffic generated through mobile phones is much less than in fixed networks.
Other related wireless technologies have experienced growth similar to that of cellular. For example, cordless telephony, two-way radio trunking systems, paging (one way and two way), messaging, wireless local area networks (WLANs) and wireless local loops (WLLs). In addition, new broadband communication schemes are rapidly being deployed to provide users with increased bandwidth and faster access to the Internet. Broadband services such as xDSL, short range high speed wireless connections, high rate satellite downlink (and the uplink in some cases) are being offered to users in more and more locations.
Along with the unprecedented expansion in the field of communications (e.g., radio, mobile, fiber and satellite communications, etc.), comes an increased interest in the problems associated with noise and interference. Radio interference is described as the presence of unwanted RF signals that impair radio reception and can be either accidental or intentional. In addition, interference can be caused by external means (e.g., neighboring communications systems, co-channel and adjacent interference, etc.) or by internal processes utilized in the process of communication (e.g., intersymbol interference, etc. Examples of radio interference include co-channel interference, adjacent channel interference, intersymbol interference, electromagnetic interference (EMI), splatter, harmonics, spurious emissions, spurious response, jamming and band congestion.
In mobile communications, however, four types of interference dominate. These include co-channel interference, adjacent channel interference, intersymbol interference and interference due to system nonlinearities. In co-channel interference, the interfering signal has the same carrier frequency as the desired information signal. The interfering signal usually is at a lower power level than the desired signal making reception difficult but possible. Higher power levels cause the desired signal to be jammed and reception impossible.
Adjacent channel interference occurs when a radio receiver is tuned to a particular frequency and interference is received from a signal on a nearby frequency. This type of interference is likely when an extremely strong interfering signal lies near the desired information signal. In other words, the interfering signal has a carrier frequency adjacent to the frequency of the desired information signal. Using a selective filter having a suitable bandwidth for the signal to be received while making the adjacent channel response as low as possible is one possible approach to reducing the effects of this type of interference.
In intersystem interference, the interfering signal comes from a different system, which operates either within the geographic proximity of the system or comes from another system, typically non-friendly, that generates an interfering signal of sufficient strength to totally disrupt communications. The latter type of interference is known as jamming and is usually used during wartime to disrupt enemy communications, radar, radiolocations and radio navigation. This type of interference is not very common in cellular systems.
Interference can also be caused by nonlinearities in the system and/or other effects of filters such as intermodulation distortion wherein nonlinear system components (especially in analog signal transmission) cause spurious signals, which may play a role in causing interference in adjacent channels. Intersymbol interference (ISI) in digital transmission operates to effectively lower the bit error rate (BER). ISI can be caused by the filtering of neighboring symbols using filters that do not meet the Nyquist conditions or by channel scattering, such as occurs in mobile communication.
In addition, internal circuit noise such as thermal noise, shot noise, hiss, for example, is produced by the active components making up the electronic equipment. At high frequencies, i.e. above 30 MHz, it is important that internal noise be kept as low as possible in the first stages of a multistage amplifier chain because any noise generated in one amplifier will be picked up and amplified along with the desired signals in subsequent stages. This type of noise is always present in communication systems and samples of this type of noise are typically uncorrelated, i.e., they constitute white noise.
Not all the above types of interference require special attention in the receiver of mobile communication systems. In the GSM standard, for example, the types of interference most frequently encountered include co-channel interference, adjacent channel interference and interference due to nonlinearities in the system.
In mobile radio systems, some form of equalization is required to deal with the effects of channel scattering and also with the effects of ISI caused by transmit and receive filters. An example of a commonly used type of equalizer includes the maximum likelihood sequence estimation (MLSE) equalizer that utilizes the well known Viterbi Algorithm (VA). The goal is to limit the ISI as much as possible while maintaining a reasonable complexity for the receiver.
Another important consideration in the design of the system is the degree of whiteness and Gaussian distribution of the noise sources other then ISI. For example, in the case wherein the main noise source is thermal additive white Gaussian noise (AWGN) noise, simple equalization can be used to reduce the noise. Other types of noise and interference, such as co-channel and adjacent channel interference, however, require additional treatment. The goal is to find a receive filter that maximizes the signal to interference ratio (SIR) in this case while keeping ISI at as low a level as possible. In addition, interference suppression (i.e. cancellation) should be applied in an attempt to remove the interference from the received signal before a decision about the data is made.
A diagram illustrating an example prior art communication system employing an inner and outer encoder in the transmitter, inner and outer decoding stages in the receiver and a noise source after the channel is shown in FIG. 1. The communication system, generally referenced 10, represents the typical scheme that is used in mobile radio transceivers. In such a system, the transmitter 11 comprises an encoder 14, symbol generator (i.e. bit to symbol mapper) 16 and modulator 18. Input data bits 12 to be transmitted are input to the encoder 14 which may comprise an error correction encoder such as Reed Solomon, convolutional encoder, parity bit generator, etc. The encoder functions to add redundancy bits to enable errors in transmission to be located and fixed.
It is noted that both the inner and outer decoders in the receiver have complimentary encoders in the transmitter. The outer encoder in the transmitter comprises the encoder 14, e.g., Reed Solomon, etc. The inner encoder comprises the channel 20 that often times can be modeled as an L-symbol long FIR-type channel.
The bits output from the encoder may optionally be interleaved wherein the order of the bits are changed so as to more efficiently combat burst errors. The rearrangement of the bits caused by interleaving improves the resistance to burst errors while adding latency and delay to the transmission.
The bits are then mapped to symbols by the bit to symbol mapper 16. The bit to symbol mapper functions to transform the bits to modulator symbols. For example, an 8-PSK modulator as used in GSM EDGE systems uses 8 symbols Sk (k=0 . . . 7), hence the mapper takes three bits and converts them to one of eight symbols. Thus, the bit to symbol mapper generates a symbol for every three input bits.
The output from the mapper is input to the modulator 18 which receives symbols in the M-ary alphabet and generates the analog signal that is subsequently transmitted over the channel 20. The channel may comprise a mobile wireless channel, e.g., cellular, cordless, a fixed wireless channel, e.g., satellite, or may comprise a wired channel, e.g., xDSL, ISDN, Ethernet, etc. The processing performed in the transmitter is intended to generate a signal that can be transmitted over the channel so as to provide robust, error free detection by the receiver.
At the receiver 13, the analog signal from the channel is input to front end circuitry 22 which demodulates and samples the received signal to generate received samples y(k) 21. The symbols are then input to an inner decoder 24. An example of an inner decoder is an equalizer which compensates for the ISI caused by the delay and time spreading of the channel in attempting to detect the symbols that were originally transmitted by the modulator.
Equalizers can be adapted to output hard symbol decisions or soft symbol decisions. Examples of types of commonly used hard decision equalizers include the maximum likelihood sequence estimation (MLSE) equalizer that utilize the well known Viterbi Algorithm (VA), linear equalizer and decision feedback equalizer (DFE). Examples of soft output type equalizers include Soft Output Viterbi Algorithm (SOVA) type equalizers and equalizers based on the more computationally expensive Maximum A Posteriori (MAP) algorithm.
In the case of a hard output equalizer, the output of the inner decoder comprises symbols s(k) 23 which represent hard decisions. If a soft output decoder is used, the symbols s(k) output of the inner decoder comprise soft symbol decisions. The output of the inner decoder is then input to a soft output generator (not shown) which functions to generate soft decision information.
If interleaving was employed in the transmitter, a de-interleaver (not shown) is used to restore the original order of either the symbols or the bits, depending on the type of de-interleaver used. The bits are then input to an outer decoder 26 which functions to locate and fix errors using the redundancy inserted by the encoder. The outer decoder generates the binary receive data ak 28.
Examples of the outer decoder include turbo decoders and convolutional decoders that utilize the Viterbi Algorithm. This class of decoders provides better performance by taking into account soft information about the reliability of the received symbol. The improved performance of the decoder cannot be realized, however, when soft information about the received symbols is not available. Note that the Viterbi algorithm is widely used in communication systems and has been adapted to perform functions including demodulation, decoding, equalization, etc. Many systems utilize the Viterbi Algorithm in both the inner and outer decoding stages.
As described above, the outer decoder, in some systems, is adapted to utilize the symbol decisions output from the inner decoder, e.g., the equalizer. Optimal decoders, however, require soft decisions rather than hard decisions. For example, an outer decoder that utilizes the Viterbi Algorithm to perform convolutional forward error correction decoding, requires soft decisions as input. The advantage of a Viterbi decoder is that it can efficiently process soft decision information. In order to provide soft symbol decisions, the inner decoder typically comprises a soft output equalizer such as a SOVA or MAP based equalizer.
A drawback of the above described mobile radio system is that it lacks a mechanism for efficiently combating the types of interference most likely to effect mobile radio systems, such as co-channel interference, adjacent channel interference and interference caused by nonlinearities such as from transmit and receiver filtering.
It is desirable, therefore, to have a mobile radio receiver that is capable of reducing the types of interference that are likely to cause the most performance degradation in the receiver while achieving at the same time a minimum ISI. Ideally, the filter dynamically adjusts its characteristics in accordance with the type of interference detected in the received signal. Thus, the receiver should maximize the signal to interference ratio while keeping ISI at a minimum.
Accordingly, the present invention provides a novel and useful apparatus for and method of interference reduction in a communications receiver. The present invention is suitable for use with a wide range of channels and types of interference and is particularly useful in reducing the interference encountered in GSM, EDGE and other types of cellular channels. These types of channels are typically characterized by rapidly changing impulse response and the invention is operative to model the interference frequently encountered in such channels, generate an estimate of the noise based on the model and to subtract the estimated noise from the receive signal.
To aid in illustrating the principles of the present invention, the apparatus and method are presented in the context of a GSM mobile station. It is not intended that the scope of the invention be limited to the examples presented herein. One skilled in the art can apply the principles of the present invention to numerous other types of communication systems as well.
Ideally, an optimal receive filter can be found and used in the receiver for each given interference, thus resulting in significantly better performance. Since the receive filter cannot be chosen before the type of the interference present is known, a compensation or post filter is used as the next best alternative. An optimal receive filter can only be found after sampling the receive signal which is performed after the receive filter in the Rx front end. For example, if it is found that most of the noise energy is on side of the spectrum then it would be desirable to use a narrow receive filter at the input.
Thus, in accordance with a key aspect of the present invention a receive filter is found that maximizes the signal to interference ratio in the presence of various types of interference while keeping ISI as small as possible.
In accordance with the invention, the receive filter is fixed and is used for all types of interference, since it is not possible to determine the type of the interference before the receive filter. After the receive filter, however, the signal can be sampled and the samples used to find the necessary parameters to detect the type of interference present. The output of the interference detector comprises a determination of the type of interference. Thus, for each specific interference a compensation filter is used whose impulse response convoluted with the impulse response of the receive filter yields an impulse response as close to the desired response as possible.
The invention is operative to first find the channel estimate and the noise vector from the receive signal. Then, an interference detector is used to choose an appropriate compensation filter and select between adaptive and deterministic moving average models for noise prediction. After compensation filtering, a new channel estimate and noise vector is calculated. The new noise vector is used to determine noise whitening coefficients if the adaptive model is used. In the deterministic model case, the coefficients do not need to be calculated since they are already known, having been calculated a priori. The noise whitening coefficients are then used in an equalizer such as a Viterbi algorithm based equalizer. Note that the present invention is effective to improve the performance of the receiver in presence of residual interference having a well defined spectrum.
The invention comprises an interference detector that is operative to output a template chosen from among a plurality of interference templates. In addition, the detector outputs the type of template which may be either deterministic or adaptive and a metric that is indicative of the reliability of the decision. The particular interference template and its associated reliability are used to determine the coefficients for the compensation filter that is used to filter the received signal.
In accordance with the invention, the interference detector is effective to improve the calculation of the noise whitening model coefficients resulting in better noise whitening inside the non-linear sequence decision equalizer thus achieving better overall performance of the receiver.
Many aspects of the previously described invention may be constructed as software objects that execute in embedded devices as firmware, software objects that execute as part of a software application on a computer system running an operating system such as Windows, UNIX, LINUX, etc., an Application Specific Integrated Circuit (ASIC), Field Programmable Gate Array (FPGA) or functionally equivalent discrete hardware components.
There is therefore provided in accordance with the present invention a method of reducing interference in a communications receiver coupled to a channel, the method comprising the steps of determining a plurality of interference templates, each interference template comprising statistics derived from a different interference signal, generating statistics of a noise vector derived from a received signal, selecting an interference template corresponding to a best match between the noise vector statistics and each the interference template, generating noise whitening coefficients in accordance with the selected interference template and filtering the received signal utilizing a compensation filter whose filter coefficients are chosen in accordance with the selected interference template.
There is also provided in accordance with the present invention an apparatus for reducing interference in a communications receiver coupled to a channel, the method comprising the steps of template generation means for determining a plurality of interference templates, each interference template comprising statistics derived from a different interference signal, a noise vector determination mechanism operative to generate statistics of a noise vector derived from a received signal, an interference detector operative to select an interference template corresponding to a best match between the noise vector statistics and each the interference template, means for generating noise whitening coefficients in accordance with the selected interference template and a compensation filter adapted to filter the received signal, the compensation filter comprising filter coefficients choosen in accordance with the selected interference template .
There is further provided in accordance with the present invention a method of reducing interference in a communications receiver coupled to a channel, the method comprising the steps of determining a plurality of interference templates, each interference template comprising statistics derived from a different interference signal and being of either a deterministic type or an adaptive type, generating statistics of a noise vector derived from a received signal, selecting an interference template corresponding to a best match between the noise vector statistics and each the interference template, if the selected interference template is of the deterministic type, generating noise whitening coefficients utilizing previously calculated coefficients, if the selected interference template is of the adaptive type, generating noise whitening coefficients utilizing a moving average model and equalizing the received signal utilizing equalization coefficients derived from the noise whitening coefficients.
There is also provided in accordance with the present invention an apparatus for reducing interference in a communications receiver coupled to a channel, the method comprising the steps of template generation means for determining a plurality of interference templates, each interference template comprising statistics derived from a different interference signal and being of either a deterministic type or an adaptive type, a noise vector determination mechanism operative to generate statistics of a noise vector derived from a received signal, an interference detector operative to select an interference template corresponding to a best match between the noise vector statistics and each the interference template, means for generating noise whitening coefficients utilizing previously calculated coefficients if the selected interference template is of the deterministic type or utilizing a moving average model if the selected interference template is of the adaptive type and an equalizer operative to equalize the received signal utilizing equalization coefficients derived from the noise whitening coefficients.
There is still further provided in accordance with the present invention a method of reducing interference in a communications receiver coupled to a channel, the method comprising the steps of determining a plurality of interference templates, each interference template comprising statistics derived from a different interference signal and being of either a deterministic type or an adaptive type, generating statistics of a noise vector derived from a received signal, selecting an interference template corresponding to a best match between the noise vector statistics and each the interference template, generating a first plurality of noise whitening coefficients in accordance with the selected interference template, if the selected interference template is of the deterministic type, generating a second plurality of noise whitening coefficients utilizing previously calculated coefficients, if the selected interference template is of the adaptive type, generating the second plurality of noise whitening coefficients utilizing a moving average model, filtering the received signal utilizing a compensation filter whose filter coefficients are chosen in accordance with the selected interference template and equalizing the output of compensation filter utilizing equalization coefficients derived from the first plurality of noise whitening coefficients.
There is also provided in accordance with the present invention a communications receiver for receiving and decoding an M-ary transmitted signal comprising a radio frequency (RF) front end circuit for receiving and converting the M-ary transmitted signal to a baseband signal, a demodulator adapted to receive the baseband signal and to generate a received signal therefrom in accordance with the M-ary modulation scheme used to generate the transmitted signal, an interference reduction mechanism operative to determine a plurality of interference templates, each interference template comprising statistics derived from a different interference signal and being of either a deterministic type or an adaptive type, generate statistics of a noise vector derived from a received signal, select an interference template corresponding to a best match between the noise vector statistics and each the interference template, generate a first plurality of noise whitening coefficients in accordance with the selected interference template, if the selected interference template is of the deterministic type, generate a second plurality of noise whitening coefficients utilizing previously calculated coefficients, if the selected interference template is of the adaptive type, generate the second plurality of noise whitening coefficients utilizing a moving average model, a compensation filter adapted to filter the received signal and whose filter coefficients are chosen in accordance with the selected interference template, an equalizer adapted to equalize the output of the compensation filter utilizing equalization coefficients derived from the first plurality of noise whitening coefficients so as to generate a sequence of hard symbol decisions therefrom, a soft output generator adapted to generate soft output decisions from the sequence of hard symbol decisions and a second decoder adapted to receive the soft output values and to generate binary received data therefrom.
There is also provided in accordance with the present invention a computer readable storage medium having a computer program embodied thereon for causing a suitably programmed system to reduce interference noise from within a receive signal by performing the following steps when such program is executed on the system determining a plurality of interference templates, each interference template comprising statistics derived from a different interference signal and being of either a deterministic type or an adaptive type, generating statistics of a noise vector derived from a received signal, selecting an interference template corresponding to a best match between the noise vector statistics and each the interference template, generating a first plurality of noise whitening coefficients in accordance with the selected interference template, if the selected interference template is of the deterministic type, generating a second plurality of noise whitening coefficients utilizing previously calculated coefficients, if the selected interference template is of the adaptive type, generating the second plurality of noise whitening coefficients utilizing a moving average model, filtering the received signal utilizing a compensation filter whose filter coefficients are chosen in accordance with the selected interference template and equalizing the output of compensation filter utilizing equalization coefficients derived from the first plurality of noise whitening coefficients.